Filament joint for incandescent lamps



Nov. 22, 1960 K. s. G. PERTWEE FILAMENT JOINT' FOR INCANDESCENT LAMPSFiled Nov. 14, 1957 lnven' tov: Kenne th S. G. Per twe 5 His A t vnes.

United States Patent i FILAMENT JOINT FOR IN CANDESCENT LAMPS Kenneth S.G. Pertwee, Cleveland, Ohio, assignor to Gen eral Electric Company, acorporation of New York Filed Nov. 14, 1957, Ser. No. 696,509 6 Claims.(Cl. 313-471) My invention relates to electric incandescent lampscomprising a sealed bulb or envelope containing a tungsten filamentconnected to current supply conductors through which current is conveyedfrom terminals at the exterior of the bulb. More particularly myinvention relates to the joint or connection between the ends of thefilament and the current supply conductors.

The filaments of incandescent lamps are generally connected to thecurrent supply or lead-in conductors by a mechanical joint as opposed toa welded joint which tends to cause embrittlement and weakening of thetungsten filament adjacent to the joint, with consequent breakage. Incertain types of lamps a satisfactory joint is made simply by securelyclamping a hook in the end of the current supply conductor about the endof the filament. Howover, in other lamp types, particularly higherwattage lamps, such a joint is not feasible because the conductor isnecessarily of larger s'ze and is usually also made from more refractoryand harder metal. The art has therefore resorted to joints of rathercomplicated structure in order to secure a satisfactory mechanical andelectrical connection. However, in certain cases the frictionalconnection is unsatisfactory since it permits the filament to move andalso permits a certain amount of arcing and local overheating at thejoint. Such arcing produces a noise which, in the case of projectionlamps used in movie or television studios, is picked up by nearbymicrophones.

It is therefore an object of the present invention to provide animproved filament joint wherein the filament is firmly bonded to thecurrent supply conductor to provide a rigid connection and goodelectrical conduction without welding and without the introduction ofmaterials deleterious to the operation of the lamp.

In accordance with one aspect of my invention, the filament is joined atits ends to current supply conductors of refractory metal such asmolybdenum, tungsten, tantalum or columbium (niobium) and the joint isfirmly bonded by applying thereto an intimate mixture of fine tungstenpowder and fine platinum powder preferably made into a paste with asuitable temporary binder which is dispersed by decomposition orvolatil-ization upon heating. The joint is then heated in anon-oxidizing atmosphere at a temperature above the melting point ofplatinum (1763 C.) but below the melting point of the filament or thecurrent supply conductor. The platinum particles are thereby melted andjoined firmly to all adjacent surfaces of the tungsten filament and therefractory metal current supply conductor and form an alloy therewith.At the same time, the mixture becomes consolidated and, by alloying, themeeting point of the mixture is raised considerably so that it does notmelt during subsequent operation of the filament at incandescence. Forthis reason, it is impracticable to employ a preformed alloy of tungstenand platinum; its melting point would be so high as to make itimpractical to melt the alloy without damage to the filament or currentsupply conductor. Also, a bond employing no platinum and consisting of asuspension of molybdenum or tantalum 2,961,568 Patented Nov. 22, 1960"ice powder in a suitable binder is not entirely satisfactory since ittends to flake off and contaminate the tungsten filament.

I have found that good results are obtained with a mixture of tungstenand about 515% platinum by weight, preferably about 10% platinum andtungsten. A lesser amount of platinum does not give a sufficientlystrong bond because the mixture flakes o-if. Amounts of platinum inexcess of 15% are not necessary and would therefore unduly increase theexpense and would also reduce the melting point of the alloy. Althoughthere is a firm consolidation of the mixture and alloying of theplatinum with adjacent surfaces of the filament and current supplyconductor as well as with tungsten particles in the mixture, theresultant reaction product is probably not a homogeneous alloy. On theother hand, there is apparently no free metallic tungsten powder in theproduct although some of the tungsten particles may have only a layer ofplatinum alloyed at the surface thereof. At any rate, the resultant bondassumes a shiny metallic surface appearance and forms a firm and rigidconnection of good electrical conductivity.

Good results may be obtained with platinum powder of about 150 mesh orfiner and tungsten powder of about 200 mesh or finer. The binder may bea cellulosic material such as a solution of ethyl cellulose ornitrocellulose. Satisfactory results have been obtained with a mixtureof grams of powder consisting of 90 grams tungsten powder and 10 gramsplatinum powder thoroughly milled to form an intimate mixture to whichis added 20 cc. of a solution of ethyl cellulose in butyl acetate havinga viscosity of about 50 to 60 seconds in a No. 7 du Pont cup, orapproximately 50 centipo-ises. The viscosity may be adjusted by additionof solvent to provide a fairly thin paste of a viscosity suitable forapplication by means of a brush.

The accompanying drawing illustrates an embodiment of the invention, byway of example, in an otherwise conventional high wattage projectionlamp. In the drawing, Fig. 1 is an elevation of the lamp and Fig. 2 is afragmentary view of the filament joint on an enlarged scale.

Referring to the drawing, the lamp illustrated therein comprises asealed glass bulb or envelope 1 containing an inert gas filling andhaving a reentrant stem 2 through which is sealed a pair of currentsupply or lead-in conductors 3. The filament 4 may be of the monoplaneor biplane types comprising a plurality of coiled sections arranged inparallel relationship in or adjacent to a single plane which alsoincludes the conductors 3. The coiled sections of the filament areserially connected by loops or bight portions by means of which thefilament is supported at the top and bottom by respective groups ofsupport wires 5 and 6 which are anchored in respective top and bottomglass bridges 7 and 8 which are, in turn, supported from the conductors3 by pairs of support wires 9 and 10.

The ends of the filament 4 are electrically connected to the conductors3, in this case by means of auxiliary current supply conductors or spuds11. The spuds illustrated herein are of the type disclosed in Patent2,449,- 679, Van Horn, and each consists of a length of rigid wire 12overwound with a finer wire 13 at a pitch corresponding to that of ashort coiled leg portion 14 at the end of the filament. The Wires 12 and13 are preferably made of molybdenum although wires of tungsten,tantalum or columbium may also be employed. One end of the compositespud 11 is screwed into the coiled leg 14 and the other leg isconnected, preferably by welding, to a conductor 3. While such jointshave been used successfully in many applications, yet they have beenfound not to be wholly satisfactory at times since they have allowed thefilament to move and permitted a certain amount of arcing and localoverheating.

In accordance with the present invention, the joint is materiallyimproved and looseness is eliminated by application of the bondingmaterial described above and indicated at 15 (Fig. 2). The pastecomposed of tungsten and platinum powders and binder is applied to thejoint in any suitable manner. For example, a rather thin paste isapplied by brushing over the joint; a mere touch of the brush atopposite sides of the joint is sufficient, preferably at the end of thespud 11 where it is screwed into the coiled leg 14. The joint is thenheated in a nonoxidizing environment, preferably a reducing atmosphere,to a temperature above the melting point of platinum, preferably toabout 1850 C.

The heating of the joint may be carried out in any suitable manner. Forexample, the heating may be accomplished during the conventionalflashing treatment in a hydrogen bottle wherein the filament, mounted inplace on the lead-in conductors 3 and before sealing of the bulb 1 tothe stem 2, is subjected to a cycle of applications of increased voltagewhile disposed in a bottle containing an atmosphere of hydrogen. Duringthe flashing cycle the binder is dispersed and the platinum particles inthe bonding mixture 15 melt and bond firmly to all adjacent tungsten andmolybdenum surfaces to securely hold the spud 11 and filament leg 14together. The mixture becomes consolidated and the platinum alloys withportions of the filament leg 14, spud wires 12 and 13 and tungstenparticles in the mixture. Alternatively, the sub-assembly of filament 4and spuds 11 may be set in a jig and placed in a hydrogen furnace andfired at a temperature and for a time sufficient to melt the platinum,for instance for 3 minutes at about 1850 C.

The resulting bond firmly holds together the filament end leg 14 and thecurrent supply conductor or spud 11 throughout the life of the lampwithout the introduction of any contaminants which might adverselyaffect operation of the lamp. In fact, tests have shown an actualimprovement in maintenance of light output during life, as compared withsimilar standard lamps in which the bonding material was not employed,amounting to about 3% more light after burning for 50% of normal life.This is probably due to a reduction in blackening by the elimination ofarcing at the filament joint.

It will be understood that the structure of the filament connection orjoint may be widely varied and satisfactorily bonded by theplatinum-tungsten mixture. For example, good results have been obtainedwhen the overwind wire 13 was eliminated, the spud then being simply awire 12 of diameter suitable to fit tightly in the coil leg 14 andfirmly bonded thereto by the fusion of the platimum-tungsten mixture.Moreover, the use of auxiliary spuds may be entirely dispensed with andthe ends of the filament coiled around the conductors 3 and bondedthereto by the platinum-tungsten mixture when those conductors are madeof a refractory metal such as molybdenum.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In an electric incandescent lamp, the combination of a current supplyconductor of metal of the class consisting of molybdenum, tungsten,tantalum and columbium, a tungsten filament joined at one end to saidconductor and bonded thereto by the heat reaction product of an intimatemixture of finely divided particles of tungsten and to 15 percent byweight platinum.

2. In an electric incandescent lamp, the combination of a current supplyconductor of metal of the class consisting of molybdenum, tungsten,tantalum and columbium, a tungsten filament joined at one end to saidconductor and bonded thereto by the heat reaction product of an intimatemixture of finely divided particles of tungsten and about 10% by weightplatinum.

3. In an electric incandescent lamp, the combination of a current supplyconductor of molybdenum, a tungsten filament joined at one end to saidconductor and bonded thereto by the heat reaction product of an intimatemixture of finely divided particles of tungsten and 5 to 15 percent byweight platinum.

4. The method of bonding a joint between an incandescible tungstenfilament and a current supply conductor of metal of the class consistingof molybdenum, tungsten, tantalum and columbium which comprises applyingto the joint a paste composed of an intimate mixture of finely dividedparticles of tungsten and platinum in a heat-decomposable binder and inproportions by weight of between 5 and 15 percent platinum and 95 topercent tungsten, and heating the joint in a non-oxidizing environmentto a temperature above the melting point of platinum but below themelting points of the said conductor and filament and for a timesufficient to decompose the binder, melt the platinum particles andsinter together the tungsten particles whereby the platinum alloys withthe adjoining surfaces of the conductor, filament and tungsten particlesto form a firm bond.

5. The method of bonding a joint between an incandescible tungstenfilament anda current supply conductor of metal of the class consistingof molybdenum, tungsten, tantalum and columbium which comprises applyingto the joint a paste composed of an intimate mixture of finely dividedparticles of tungsten and platinum in a heat-decomposable binder and inproportions by weight of about tungsten and 10% platinum, and heatingthe joint in a non-oxidizing environment to a temperature above themelting point of platinum but below the melting points of the saidconductor and filament and for a time sufficient to decompose thebinder, melt the platinum particles and sinter together the tungstenparticles whereby the platinum alloys with the adjoining surfaces of theconductor, filament and tungsten particles to form a firm bond.

6. The method of bonding a joint between the coiled end portion of anincandescible tungsten filament and a portion of a molybdenum currentsupply conductor fitted closely in the said coiled end portion of thefilament, which comprises applying to the joint a thin paste con sistingof an intimate mixture of finely divided particles of tungsten andplatinum in a heat-decomposable cellulosic binder and in proportions byweight of 5-15% platinum and -85% tungsten, and heating the joint in anon-oxidizing atmosphere to a temperature of about 1850 C. for a timesufficient to decompose the binder, melt the platinum particles andsinter together the tungsten particles whereby the platinum alloys withthe adjoining surfaces of the conductor, filament and tungsten particlesto form a firm bond.

References Cited in the file of this patent UNITED STATES PATENTS2,039,772 Braselton May 5, 1936 2,258,158 Lowry Oct. 2, 1941 2,300,959Pironi Nov. 3, 1942 2,341,716 Herdman Feb. 15, 1944 2,434,478 Allen Jan.13, 1948 2,449,679 Van Horn Sept. 21, 1948 2,462,837 Braunsdorlf Mar. 1,1949 2,464,591 Larsen Mar. 15, 1949 2,844,868 Cline July 29, 1958

